This invention relates to cooling devices particularly useful for cooling to low temperatures, and, more particularly, to a cooling system utilizing a pulse tube expander having no moving parts.
Some types of sensors and electronic devices are not practically operable at temperatures above about 50.degree.-75.degree. K. and therefore must be cooled into this temperature range for proper operation. The cooling into this mid-cryogenic range is readily accomplished in some settings, such as a laboratory or a stationary service application, using a reservoir of a cryogenic fluid. In field operations, however, it is often not practical to supply a reservoir of the cryogenic fluid, and thermodynamic-cycle cooling devices must be used.
Several types of thermodynamic-cycle cooling devices are known. One commonly used cooler is based upon the Joule-Thompson gas-expansion principle. Another cooler is the pulse tube expander, based upon a modified Stirling cycle, in which pressurized gas in a regenerator/pulse tube assembly is rapidly pulsed such that compression work is done in the warm region of the assembly to remove heat from the expander, and expansion work is done in the cold region to absorb a thermal load. Pulse tube refrigeration devices are reasonably efficient, have minimal vibration, are dependable over long service lives, and are of moderate cost. The present invention deals with a cooling system utilizing an improved form of the pulse tube expander.
A basic pulse tube expander refrigerator has a regenerator linearly in series with the pulse tube, producing quite a long structure. One design modification to make the device more compact and simplify the heat rejection is a concentric design wherein a pulse tube of relatively small diameter is positioned inside and along the cylindrical axis of an annular regenerator of larger diameter. This design halves the total length of the linear device and colocates the heat rejection spaces within a single heat exchanger block, thereby simplifying the thermal management. In such a design, the regenerator must be radially insulated from the pulse tube, so that each component maintains its optimal axial temperature profile.
Most commonly, the insulation is provided by a vacuum gap between the outer wall of the pulse tube and the inner wall of the regenerator. While such a design is operable, its fabrication is relatively expensive due to the need for careful sealing of the vacuum structure and the numerous components involved. Vacuum ports must also be provided and sealed after the vacuum space is evacuated. If a vacuum leak is present or develops during service, the performance of the concentric pulse tube expander gradually degrades. Such degradation is a particular concern where the pulse tube expander must be operable continuously over a long life or after extended storage periods, a circumstance often encountered in the cooling of sensors.
There is a need for an improved pulse tube expander that has the advantages of other pulse tube expanders, but is both compact and not susceptible to degradation effects such as experienced in the conventional concentric design. The present invention fulfills this need, and further provides related advantages.